1. Field of the Invention
The present invention relates to an electro-optical element composed of nematic liquid crystal compositions having pleochroic dyes dissolved therein.
More particularly, it pertains to an electro-optical element making use of compositions of nematic liquid crystals containing at least one species of dyestuff represented by the undermentioned chemical formulae: ##SPC2##
Wherein R.sub.1 and R.sub.2 are radicals chosen from the group consisting of alkyl radical having from 5 to 20 and from 4 to 20 carbon atoms respectively.
2. Description of the Prior Art
In general, a liquid crystal device is usually composed of two parallel flat plates whose inner surfaces are coated with thin electrodes. Said plates are held at a distance of several microns apart e.g. several tens of microns, and in the space between said plates there is contained a liquid crystal composition. The optical characteristics of this device may be controlled by means of an electric or magnetic field.
When an electric field is applied to a layer of a nematic liquid crystal having a pleochroic dye dissolved therein; the layer exhibits turbulent motion or the molecules in the layer are orientated along the direction of the electric field and the dye molecules exhibit co-operative behavior with the nematic liquid crystal molecules, whereby a change in the optical absorption of the device is obtained.
In order to increase the optical absorption change of the device, it is desirable that the nematic liquid crystal layer have homeotropic, homogeneous or twisted structure, wherein the arrangement of the nematic liquid crystal is governed, according to the differences in the dielectric anisotropy of the nematic liquid crystal and the direction of the applied electric field.
For example, when using nematic liquid crystals having negative dielectric anisotropy, the dye molecules are placed in homeotropic structure (a structure inwhich the long axis of the liquid crystal molecules is aligned perpendicular to the vessel wall), and when using nematic liquid crystals having positive dielectric anisotropy, they are placed in the homogenous or twisted structure (a structure in which the long axis of the liquid crystal molecules is aligned parallel to the vessel wall).
In the liquid crystal device, where the nematic liquid crystal molecules are arranged in a predetermined direction, the molecular axis of the liquid crystal can be controlled to turn a maximum of 90.degree. by applying an electric field.
As the dye molecules are oriented co-operatively with the alignment of the nematic liquid crystal materials in the electric field, a display with high contrast can be obtained in the aligned phase of a nematic liquid crystal.
As dyes, azo, anthraquinone, azine, xanthene and related coloring matter and other dyes which are soluble in the liquid crystalline substances are available.
Of these dyes, some induce a change in the optical absorption of the device, and some do not, when they are aligned with the liquid crystal, and an electric field is imposed on them.
The dyes may be further classified by four different functions based on the way in which the change of optical absorption of the device (the change of state from that when the electric field is not applied to that when the electric field is applied takes place; as follows:
1. Dyes which change from the colorless state to colored state.
2. Dyes which change from the colored state to colorless state.
3. Dyes which change from a colored state to another colored state showing a different hue.
4. Dyes which do not undergo the change of state when the electric field is applied.
Displays with color to color contrast can be achieved by mixing a plurality of dyes which have different hue and different function in display performance.
Most of the customary pleochroic dyes are long cylindrically shaped molecules that show little or no absorption of light in the direction along their long axis and absorption of light in the direction along their short axis. When such dyes are placed in the nematic liquid crystal layer having a homeotropic structure at an appropriate concentration such that no unoriented molecules remain, the transmitted light appears to the observer in the colorless state with no applied field and in the colored state when an orientating field is applied.
Moreover, when they are placed in a nematic liquid crystal layer having an homogeneous or twisted structure, the transmitted light appears to the observer in the colored state with no applied field and in the colorless state when an orienting field is applied.
The dyes of this kind include compounds such as: ##SPC3##